Such a control is described in DE 39 37 290 A1. The ionization electrode is located in a d.c. circuit there. The evaluation of the ionization current is problematic in practice if a proportional relationship is to be determined between the ionization current and the lambda value.
A control device for a gas blower burner is described in the older patent application P 44 33 425. The ionization current can be reliably evaluated by superimposing an a.c. voltage. The current air excess (lambda value) of the current state of combustion is determined by means of an ionization electrode and is compared with a set point set in the control circuit. The composition of the gas-combustion air mixture is adjusted correspondingly, so that a desired lambda set point is maintained as an end result. A superstoichiometric ratio of air to gas is desired, and the lambda set point is preferably between 1.15 and 1.3. It is achieved as a result that optimal combustion takes place in terms of the emissions and the firing technical efficiency with different types of gas, e.g., natural gas and liquefied gas, and under varying ambient conditions.
The thermal coupling between the ionization electrode and the gas burner may change during the operation, e.g., due to bending, wear and contamination of the ionization electrode or fouling of the burner. This was found to lead to changes in the ionization current and consequently in the measured variable derived from it despite a constant lambda value. Consequently, the proportionality factor between the lambda value and the electrical variables derived from it changes. Since this changed measured voltage is present at the comparator of the control circuit, on which the set point, which is unchanged, also acts, the control circuit will adjust the gas-to-air mixture, i.e., the lambda value, as a result of which a deviation of the actual lambda value from the lambda set point will take place, which is undesirable.